1. Field of the Invention
The present invention relates to a multi-wavelength semiconductor laser device, and more particularly to a multi-wavelength semiconductor laser device capable of simultaneously or selectively oscillating laser light of three different wavelengths (e.g., 460 nm, 530 nm and 635 nm), and a method for producing the multi-wavelength semiconductor laser device.
2. Description of the Related Art
In general, a semiconductor laser device is one that produces light amplified by stimulated emission of radiation. The light produced by the semiconductor laser device has a narrow frequency width (one of short-wavelength characteristics), superior directivity and high output. Due to these advantages, the semiconductor laser device is used as a light source for an optical pick-up apparatus of an optical disc system, such as a CD (compact disc) or DVD (digital video disc) player, as well as, is widely applied to a wide range of fields of optical communications multiplex communications, space communications and the like.
In recent years, a multi-wavelength semiconductor laser device capable of oscillating two or more different wavelengths has been required in the field of optical discs using laser as a light source for writing and reading information. For example, a two-wavelength semiconductor laser device is currently developed as a light source for both CD players having a relatively low data density and DVD players having a relatively high data density.
FIGS. 1a to 1g are cross-sectional views illustrating the overall procedure of a conventional method for producing a two-wavelength semiconductor laser device.
Referring to FIG. 1a, a first semiconductor laser epitaxial layer oscillating light at a wavelength of 780 nm is formed on an n-type GaAs substrate 11. Specifically, the first semiconductor laser epitaxial layer is formed by sequentially growing an n-type AlGaAs clad layer 12a, an AlGaAs active layer 13a and a p-type AlGaAs clad layer 14a on the GaAs substrate 11.
Thereafter, the first semiconductor laser epitaxial layer, including the layers 12a, 13a and 14a, is selectively removed by photolithography and etching to expose a portion of a top surface of the GaAs substrate 11, as shown in FIG. 1b. 
Next, as shown in FIG. 1c, a second semiconductor laser epitaxial layer oscillating light at a wavelength of 650 nm is formed on the exposed portion of the GaAs substrate 11 and the unremoved portion of the first semiconductor laser epitaxial layer. Specifically, the second semiconductor laser epitaxial layer is formed by sequentially growing an n-type AlGaInP clad layer 12b, a GaInP/AlGaInP active layer 13b and a p-type AlGaInP clad layer 14b. 
Thereafter, the second semiconductor laser epitaxial layer, including the layers 12b, 13b and 14b, formed on the first semiconductor laser epitaxial layer is removed by photolithography and etching, and at the same time, the first epitaxial layer is separated from the second epitaxial layer, as shown in FIG. 1d. 
Next, as shown in FIG. 1e, the p-type AlGaAs clad layer 14a and the p-type AlGaInP clad layer 14b are selectively etched by a common process to form ridge-shaped layers 14a′ and 14b′, which contribute to an improvement in current injection efficiency. Then, as shown in FIG. 1f, n-type GaAs current-limiting layers 16a and 16b and p-type GaAs contact layers 17a and 17b are formed.
Finally, as shown in FIG. 1g, p-side electrodes 19a and 19b formed of Ti, Pt, Au or an alloy thereof are formed on the p-type GaAs contact layers 17a and 17b, respectively, and then an n-side electrode 18 formed of Au/Ge, Au, Ni or an alloy thereof is formed on a bottom surface of the GaAs substrate 11 to produce the two-wavelength semiconductor laser device 10.
In this manner, the semiconductor laser device 10 oscillating light of two different wavelengths is produced on a single substrate, enabling integration into one chip. Accordingly, the conventional method is advantageous compared to a method wherein respective semiconductor laser devices are separately produced, and are then attached to one substrate by die bonding, in terms of the following advantages: i) the separate production and bonding processes are omitted, thus shortening the overall production procedure, and ii) poor alignment caused during die bonding of chip can be solved.
As explained earlier in FIGS. 1a to 1g, the conventional method is limited to the two-wavelength (650 nm and 780 nm) semiconductor laser device, and thus cannot be applied to a three-wavelength (further including light of a short wavelength) semiconductor laser device. For example, two laser structures composed of nitride epitaxial layers oscillating light at wavelengths of 460 nm and 530 nm, and one laser structure composed of an AlGaInP-based epitaxial layer oscillating light at a wavelength of 635 nm are required to produce a multi-wavelength semiconductor laser device oscillating red, green and blue light. In this connection, there is a problem that since GaN-based epitaxial layers are particularly required to produce a semiconductor laser device oscillating light at wavelengths of 460 nm and 530 nm, they cannot be formed on the same substrate, together with a semiconductor laser structure oscillating light at a wavelength of 635 nm.
More specifically, since there is a large difference in the lattice constant between the AlGaInP epitaxial layer (about 5.6 Å) and the GaN epitaxial layer (about 3.2 Å) for the semiconductor laser structure oscillating light at a wavelength of 635 nm, it is difficult to grow the AlGaInP and GaN epitaxial layers on the same substrate. The AlGaInP epitaxial layer can be formed with superior crystallinity on a GaAs substrate, whereas the GaN epitaxial layer can be formed with superior crystallinity only on substrates for growth of a nitride semiconductor, such as GaN, sapphire and SiC substrates. Consequently, a multi-wavelength semiconductor laser device oscillating three-color light, for example, at wavelengths of 460 nm, 530 nm and 635 nm, cannot be substantially produced by the conventional method for producing a two-wavelength semiconductor laser device.